In a recording-disc information reading and reproducing apparatus of, for example, the optically or electrostatically scanning type, multiplex signals consisting of frequency modulated video and/or audio carriers superposed on each other are stored in the form of a series of depressed areas or "pits" formed in each or one information-carrying face of a video or audio disc. These depressed areas or pits are arranged in a spital track or in a number of concentric tracks about the center axis of the disc. The video and/or audio information thus stored in the recording disc is read out by optically, electrostatically or otherwise scanning the individual pits along the spiral track or each of the concentric tracks. In the case of an optically scanning video or audio information reading and reproducing apparatus, for example, the video and/or audio information stored on a video or audio recording disc is read out by scanning the pits by means of a beam of laser light and thereby detecting the lengths of and spacings between the scanned pits. During playback of such a recording disc or during selection of desired pieces of information out of the information carried on the information-carrying face of the disc to be played back, the disc is driven for rotation about the center axis thereof and the beam of the laser light is displaced radially of the information-carrying face of the rotating disc. The laser beam thus directed onto a target track on the information-carrying face of the disc is reflected from the face or passed through the disc and the information picked up by the laser beam from the information-carrying face of the disc is converted into electric signals. These electric signals are further converted upon frequency demodulation into video and/or audio signals to be reproduced.
The laser beam to read out the information recorded on an information-carrying face of a video or audio disc is transmitted from a laser emitter to a tracking mirror through a lens system and is reflected from the tracking mirror toward the information-carrying face of the disc to be played back or scanned by the beam. The tracking mirror is turnably mounted on a slider movable back and forth in a radial direction of the disc to be played back or scanned and is urged to stay in a predetermined neutral or home angular position about the axis of rotation thereof on the slider. During scanning of the video or audio disc to be played back, the slider is driven to travel in such a direction with respect to the disc and, concurrently, the tracking mirror is driven to turn between two opposite limit angular positions from the neutral or home angular position thereof about the axis of rotation of the mirror on the slider. These motions of the slider and the tracking mirror are effected under the control of a tracking servo system.
A conventional tracking servo system for use in a recording-disc information reading and reproducing apparatus has, in its control servo loop, error signal producing means adapted to produce a tracking error signal which is continuously variable in magnitude with an amount of deviation, if any, of the scanning spot of the laser beam or any other form of pickup medium or a detecting spot from a target track on the disc to be scanned.
It is, in this instance, known to those skilled in the art that the amplitude of such a tracking error signal has a tendency to vary depending upon the locations of the pickup medium with respect to the center axis of the recording disc being scanned. That is, the amplitude of the tracking error signal tends to decrease as the radial displacement of the pickup medium from the center axis increases. If the tracking error signal thus variable with the distance in radial direction between the axis of rotation of the rotating disc and the pickup medium scanning the disc is used directly as the signal to control the tracking servo system, there would result deterioration in the performance reliability of the tracking servo system per se. The present invention contemplates elimination of such a drawback inherent in a prior-art tracking servo system of a recording-disc information reading and reproducing apparatus.
It is, accordingly, an important object of the present invention to provide an improved tracking servo system which will enable the pickup medium or detecting spot of a recording-disc information reading and reproducing apparatus to accurately scan a recording disc throughout the radial displacement of the pickup medium or detecting spot with respect to the recording disc during scanning of the disc.